Bede's Ecclesiastical History of England, ed. by A.M. Sellar, [1907], at sacred-texts.com

CHAP. X. How, in the reign of Arcadius, Pelagius, a Briton, insolently impugned the Grace of God. [395 AD]

In the year of our Lord 394, Arcadius, the son of Theodosius, the forty-third from Augustus, succeeding to the empire, with his brother Honorius, held it thirteen years. In his time, Pelagius, [Pelagius, the founder of the heresy known as Pelagianism, was probably born in 370 A.D., and is said to have been a Briton. His great opponent, St. Augustine, speaks of him as a good and holy man; later slanders are to be attributed to Jerome’s abusive language. The cardinal point
in his doctrine is his denial of original sin, involving a too great reliance on the human will in achieving holiness, and a limitation of the action of the grace of God] a Briton, spread far and near the infection of his perfidious doctrine, denying the assistance of the Divine grace, being seconded therein by his associate Julianus of Campania, who was impelled by an uncontrolled desire to recover his bishopric, of which he had been deprived. St . Augustine, and the other
orthodox fathers, quoted many thousand catholic authorities against them, but failed to amend their folly; nay, more, their madness being rebuked was rather increased by contradiction than suffered by them to be purified through adherence to the truth; which Prosper, the rhetorician, has beautifully expressed thus in heroic" verse :—

"They tell that one, erewhile consumed with gnawing spite, snake-like attacked Augustine in his writings. Who urged the wretched viper to raise from the ground his head, howsoever hidden in dens of darkness? Either the sea-girt Britons reared him with the fruit of their soil, or fed on Campanian pastures his heart swells with pride."